The present invention relates to wireless communications and more particularly to cooperative communications. Future wireless communication environments are highly resource-constrained, offering a limited and tightly regulated spectrum. The energy supply on wireless terminals is usually limited, and, in many scenarios they must operate for many years without battery replacement. A promising approach to overcome such limitations is the use of multiple antennas both to transmit and receive information, which can provide a diversity gain as well as a multiplexing gain at no extra bandwidth or power consumption [1]-[7]. Although attractive, this option requires collocated antenna elements with antenna spacing of tens of wavelengths at the base station and up to a wavelength at the terminal. In many practical scenarios, space limitations at the terminal site make antenna spacing critical, a physical constraint that significantly limits the applicability of antenna arrays.
A series of recent papers has suggested a new form of diversity obtained from virtual antenna arrays consisting of a collection of distributed antennas belonging to different wireless terminals [8]-[14]. We refer to these types of communications as cooperative communications. With cooperative communications, the source broadcasts its data to both the relay and the destination.
The key property of such cooperative communications is that transmitted signals can, in principle, be received and processed by any node. Nodes can act as relays and help other nodes, either individually or in groups. Furthermore, nodes can create additional paths for a source-destination pair to increase diversity against fading and interference, and allow spatial multiplexing between other nodes. Despite the advantage of present day examples of cooperative communications, problems remain.
Wireless ad hoc networks consist of a number of terminals (nodes) communication on a peer-to-peer basis, without the assistance of centralized infrastructure. As the amount of energy required to communicate reliably over wireless channels increases rapidly with distance, multi-hop communication has been vastly favored over a long-range single-hop link in wireless ad-hoc networks. Energy efficiency is a key design objective in most of the research related to wireless ad-hoc networks because the nodes are energy constrained. However, besides energy efficiency, hardware complexity is another important aspect that needs to be taken into account when designing wireless ad-hoc networks.
Recently, cooperative relaying is gaining significant attention in this perspective. This approach employs several nodes as relays for an active source/destination pair and use multiple relay nodes as a virtual (or distributed) antenna array to realize space-time coding or MIMO system in a distributed fashion. To date, the cooperative relaying techniques have primarily been proposed to achieve diversity gains [1]-[10]. These approaches, known as cooperative diversity (C-DIV), can improve detection reliability; however, spectral efficiency (rate) is usually sacrificed for diversity gain (reliability). In particular, when high-rate data is being delivered to a destination, the hardware complexity and energy consumption on each relay node can be substantial because each relay needs to detect and forward a packet at the same rate as the source generates it.